Q. Write short notes on cloud and pour point of lubricant.
Ans. Cloud and pour point of lubricant: When the temperature is lowered up to a particular point, the oil becomes cloudy in appearance. This temperature is called cloud point. When the temperature is further lowered up to a certain temperature, the oil ceases to flow or pour or becomes semisolid. This temperature is known as pour point. Important example of cloud and pour points are refrigerator plants and aircraft engines, which may be required to start and operate at and sub zero temperatures. The apparatus employed for the determination of pour point is pour point apparatus. It consists of following parts:
1. Flat bottomed tube: It is made of glass and has standard dimensions.
2. This tube is enclosed in a glass or metal jacket which is firmly fixed in a cooling bath having suitable freezing mixture.
Working: Test oil is filled inside the tube. Thermometer is adjusted inside the oil and in the cooling bath and cork bath is fitted to the mouth of flat bottomed tube. The tube is then kept in the freezing mixture. The freezing mixture is prepared as below:
Ice + water
Up to + 100C
Ice + NaCl
Up to - 100C
Ice + CaCl2
Up to - 250C
Solid CO2 + Acetone
Up to - 600C
With every degree fall of temperature of oil, the tube is taken out and observed for a moment. The temperature, at which hazy appearance give cloud points. Cooling process is continued till oil stop flowing. That temperature is pour point.